1. Field of the Invention
The present invention relates generally to focal plane arrays and, more particularly, to microbolometer focal plane arrays.
2. Related Art
A microbolometer, which detects infrared radiation, is well known in the art. Modern microbolometer structures are typically fabricated on monolithic silicon substrates to form an array of microbolometers, with each microbolometer functioning as a pixel to produce a two-dimensional image. The change in resistance of each microbolometer is translated into a time-multiplexed electrical signal by circuitry known as the read out integrated circuit (ROIC). The combination of the ROIC and the microbolometer array is commonly known as a microbolometer focal plane array (FPA) or microbolometer infrared FPA. Microbolometers are described in further detail in U.S. Pat. Nos. 5,756,999 and 6,028,309, which are herein incorporated by reference in their entirety.
Microbolometer array performance is typically degraded due to non-uniform responses among the individual microbolometer detectors to uniform incident infrared radiation. Factors contributing to the performance degradation include variations in the infrared radiation absorption coefficient, resistance, temperature coefficient of resistance (TCR), heat capacity, and thermal conductivity of the individual detectors. Because the magnitude of the non-uniformity can be substantially larger than the magnitude of the actual response due to the incident infrared radiation, various techniques are typically required to compensate for the non-uniformity and acquire the portion of the signal representing the incident infrared radiation.
FIG. 1 illustrates a conventional method for measuring the microbolometer resistance. A voltage (V) is applied across the series combination of a microbolometer 3 (that can receive incident infrared radiation 1) and a resistive load (Rload) 6. An output voltage (Vout) is measured across microbolometer 3 to determine its resistance (Rbolometer), according to the following equation.
Rbolometer=Rload/(V/Voutxe2x88x921)
The temperature rise in microbolometer 3 due to self-heating generally is significantly larger than the temperature rise resulting from the incident infrared radiation. If the voltage (V) is multiplexed or periodically applied during each sample period, the self-heating behavior is as shown in FIG. 2. The microbolometer temperature rises significantly during each sample period (also referred to as a bias period and indicated by a pulse bias spike in the figure).
One drawback of this characteristic is that the pulse bias heating causes the microbolometer 3 (also referred to as the active microbolometer and which is thermally isolated from the substrate) to operate at a different temperature than resistive load 6 (which may also be a microbolometer and referred to as the load microbolometer that is thermally shorted to the substrate). Also, various other non-uniformities or variables between microbolometer 3 and resistive load 6 may cause a difference in the TCR between the two microbolometers. Thus, the pulse bias heating and other factors contribute to a mismatch in relative TCR between resistive load 6 and microbolometer 3.
This mismatch in relative TCR limits the range of operating temperatures for the microbolometer array. For example, as shown by the graph in FIG. 3, the output voltage for a microbolometer in the microbolometer array drops below the minimum dynamic range of the system prior to reaching the maximum desired substrate temperature. Alternatively as shown in FIG. 4, the output voltage for another microbolometer in the microbolometer array rises above the maximum dynamic range of the system prior to reaching the maximum desired substrate temperature.
For a typical microbolometer array, the output voltage produced by each microbolometer may vary over substrate temperature significantly, as shown in FIG. 5, for six exemplary microbolometers from the microbolometer array. The average output voltage from a certain number of microbolometer elements exceeds the minimum and maximum signal range, as shown in the histogram in FIG. 6, resulting in unsatisfactory FPA performance within the desired temperature range of operation.
Conventional microbolometer arrays often provide a correctable output only within a small range of substrate temperatures, on the order of 0.005 to 0.025 degrees Kelvin. A thermo-electric cooler, temperature sensor, and temperature control electronics are employed to maintain the substrate temperature within this range to minimize microbolometer array non-uniformities, which adds to system cost and complexity. As a result, there is a need for techniques that address microbolometer array properties and non-uniformities over a wider range of temperatures.
Microbolometer circuitry and methods are disclosed herein. In accordance with one embodiment, microbolometer focal plane array (FPA) circuitry is disclosed that provides temperature compensation over a wide temperature range. The relative mismatch in TCR between the active microbolometers and the load or reference microbolometers is compensated to allow the removal of strict temperature stability requirements. For example, rather than requiring temperature stability of the microbolometer array to within a fraction of a degree, the operating temperature range may be expanded significantly, such as from xe2x88x9240xc2x0 to 55xc2x0 C. Methods are also disclosed for providing calibration and applying the calibration values to the microbolometer FPA circuitry and to processing of the resulting signal values from the microbolometer FPA circuitry. Thus, circuitry and methods disclosed herein overcome many of the disadvantages of the prior art, such as complex and costly cooling systems, and provide infrared technology more applicable to low-cost, high-volume commercial markets.
More specifically, in accordance with one embodiment of the present invention, a microbolometer circuit includes a first microbolometer, a variable resistor coupled to the first microbolometer, and a biasing circuit coupled to the first microbolometer or the variable resistor to provide a load current.
In accordance with another embodiment of the present invention, a method of calibrating a microbolometer detector circuit includes calibrating a variable resistor to compensate for a relative temperature coefficient of resistance between an active microbolometer and a load over a desired temperature range; and calibrating an offset for an output signal generated by the microbolometer detector circuit.
The scope of the invention is defined by the claims, which are incorporated into this section by reference. A more complete understanding of embodiments of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly.